UCB has announced FDA’s approval of Kygevvi (doxecitine and doxribtimine) for the treatment of adults and pediatric patients living with thymidine kinase 2 deficiency (TK2d) and is the first and only approved treatment for these patients living with TK2d.¹

"The approval of doxecitine and doxribtimine represents a pivotal moment for the TK2d community who previously had no FDA-approved treatment options for this rare genetic mitochondrial disease beyond supportive [palliative] care," said Donatello Crocetta, chief medical officer at UCB. "We extend heartfelt thanks to the patients, families and friends, advocates, healthcare providers and dedicated clinical trial teams who have partnered with us on this important journey."

Why did FDA approve Kygevvi?

Kygevvi’s approval is based on safety and efficacy data collected from a Phase II clinical study, two retrospective chart review studies, and an expanded access use program.¹ In total, the studies included 82 various patients treated with Kygevvi or pyrimidine nucleosides, aged 12 years and under. Efficacy was measured by comparing overall survival in pediatric and adult treated patients to an external control group of untreated patients who were matched to treated patients using age of TK2d symptom onset.¹ Trial results yielded a total of 78 matched pairs amongst participants, with results showing that survival time beginning from treatment start was improved.¹ According to a press release from UCB, results showed that treatment reduced the overall risk of death from treatment start by approximately 86% in 78 treated patients, with the median duration of treatment being 4 years, and a median dose of 762 mg/kg/day.¹

Additionally, Kygevvi was granted Orphan Drug, Breakthrough, Priority Review and Rare Pediatric Disease designations from the FDA, and with this approval by FDA, UCB was awarded a Rare Pediatric Disease Priority Review Voucher (RPDPRV) redeemable for a priority review for a future marketing application.¹

What is Kygevvi?

Kygevvi is made up of a combination of doxecitine and doxribtimine, both pyrimidine nucleosides, and is indicated for the treatment of TK2d in adults and pediatric patients with an age of symptom onset on or before 12 years.¹ Treatment using Kygevvi is intended to incorporate pyrimidine nucleosides, deoxycytidine, and deoxythymidine into skeletal muscle mitochondrial DNA, restoring mitochondrial DNA copy number.¹

What is thymidine kinase 2 deficiency (TK2d)?

TK2d is an ultra-rare, life-threatening, genetic mitochondrial disease that progresses severe muscle weakness over time, and prior to Kygevvi’s approval had no approved treatment options beyond supportive care.¹ Tk2d is often fatal, with patients experiencing initial symptoms on or before the age of 12 years facing a severe risk of premature death.¹

"I've been studying mitochondrial diseases for more than three decades and have witnessed firsthand the impact TK2d has on patients and their families. We have been waiting for an approved treatment for many years, and this approval marks a significant milestone in how we can support and manage this debilitating condition," said Dr. Michio Hirano, professor of neurology and chief of the division of neuromuscular medicine at Columbia University Irving Medical Center.

United Mitochondrial Disease Foundation president and CEO’s statement about Kygevvi’s approval

"It's hard to overstate the importance of this FDA approval for those diagnosed with TK2d. This is an ultra-rare disease community in dire need of treatment options. For too long, caregivers and their families have had to endure the burden of this disease," said Kristen Clifford, United Mitochondrial Disease Foundation president and CEO. "Having the first-ever FDA-approved therapy for TK2d in this patient population not only meets a critical medical need - it represents something greater - hope for the future."

Sources

1. U.S. FDA approves KYGEVVI (doxecitine and doxribtimine), the first and only treatment for adults and children living with thymidine kinase 2 deficiency (TK2d) UCB November 3, 2025 https://www.prnewswire.com/news-releases/us-fda-approves-kygevvi-doxecitine-and-doxribtimine-the-first-and-only-treatment-for-adults-and-children-living-with-thymidine-kinase-2-deficiency-tk2d-302603083.html